Model Organisms

C.elegans

The early embryo and the germline of C.elegans are highly regulated on the posttranscriptional level. For example, while PolII activity starts at the 4-cell stage, the earlier divisions already differentiate the future germline from the soma and set the body axes, without any transcription at all. [Stitzel and Seydoux 2007, Yamamoto, I.; Kosinski, M. E. & Greenstein, D. (2006)]

(Video by Marlon Stoeckius)

S.mediterranea

The fresh water flatworm S.mediterranea has been described already more than 200 years ago. Like many flatworms, it exhibits an astounding capacity to regenerate after injuries: a single animal, cut into dozens of pieces, may give rise to dozens of fully regenerated, clonal animals. Also, unlike another famous regeneration model organism, the radial-symmetric Hydra, the bilateral planarians are complex, with an organized nervous system, eyes and other organs.
The young Charles Darwin collected some land planarians during his voyage with the HMS Beagle (1831 to 1836). He notes:

"Having cut one of them transversely into two nearly equal parts, in the course of a fortnight both had the shape of perfect animals. I had, however, so divided the body, that one of the halves contained both the inferior orifices, and the other, in consequence, none. In the course of twenty-five days from the operation, the more perfect half could not have been distinguished from any other specimen. The other had increased much in size; and towards its posterior end, a clear space was formed in the parenchymatous mass, in which a rudimentary cup-shaped mouth could clearly be distinguished; on the under surface, however, no corresponding slit was yet open. If the increased heat of the weather, as we approached the equator, had not destroyed all the individuals, there can be no doubt that this last step would have completed in structure. Although so well known an experiment, it was interesting to watch the gradual production of every essential organ, out of the simple extremity of another animal." ["The Voyage of the Beagle" pp.26]